The present invention relates to a printing device.
Generally, in a printing device such as a one which uses a type wheel, a carriage is loaded with a printing mechanism and movable along a platen to allow the printing mechanism to print out information on a paper, which is wrapped around a platen. A printing device of the type described has various problems left unsolved, as described hereinafter.
A first problem stems from the carriage and carrier which have heretofore been implemented with die castings. Although die castings are suitable for mass-production and surface treatment such as plating and painting, they are thick and heavy and, therfore, a support shaft and a support rail adapted to support the carrier as well as side frames adapted to support the support shaft and support rail have to be considerably rigid. Besides, a bulky space motor is required to move such a thick and heavy carrier, increasing the production cost of a printing device.
A second problem is as follows. In a printing device of the type concerned, a platen is supported at both ends by side frames. A gear pulley is mounted on one of the side frames and an auxiliary pulley on the other, and a space wire is stretched between the two pulleys and connected to a carrier. The gear pulley is rotated by a space motor to move the carrier along a platen so that information may be printed on a paper, which is wrapped around the platen. The connecting position of the carrier to the space wire is a critical factor since it determines not only a printing position at which information is to be printed out on the paper but also whether the carrier is capable of moving over the entire width of the platen. It has been customary to connect the carrier to a space wire by fixing the carrier in a home position and winding the space wire around the gear pulley a predetermined number of turns. However, winding the space wire around the gear pulley a predetermined number of turns while holding the carrier in a home position as stated is undesirable since the manipulation for interconnecting the carrier and space wire is difficult and inaccurate.
A third problem is related to the orientation of a space motor. Specifically, a space motor installed in a printing device of the type described is usually mounted on a side frame in such an orientation that an output shaft thereof extends substantially horizontally. To meet a demand for a light-weight and small-size printer configuration, there has been developed a flat motor whose dimension along an output shaft thereof is short. A flat motor, however, brings about another problem that because the motor has a substantial length (width) in a direction perpendicular to the output shaft, positioning the motor such that the output shaft extends substantially horizontally invites an increase in the overall height of the printing device.
A fourth problem is derived from a prerequisite that in such a printing device, e.g. a one using a type wheel the reference position of a carriage relative to a carrier in a printing condition be set up with accuracy in order to enhance accurate printing. In a printer using a type wheel, for example, the front face of a carriage is rotated rearwardly in the event of replacement of the type wheel; in the event of printing, the carriage front face is rotated toward a platen. At this instant, should the carriage be stopped at different positions, the printing accuracy would be deteriorated. In light of this, a prior art printing device is provided with special reference setting mechansims, e.g., a reference plate mounted on a carrier and formed with a reference groove, a projection provided on a carriage to be received in the reference groove, and a mechanism for retaining the projection in the reference groove. Such a scheme cannot be embodied without adding to the cost since extra members have to be mounted on the carrier and carriage for setting up the reference print position.
A fifth problem is as follows. In a printing device of the type described, a platen is rotatably supported by right and left side frames, and the platen is rotated by a line feed motor. A carriage is movable along the platen to allow a printing mechanism mounted on the carriage to print out information on a paper, which is set on the platen. A prior art platen is produced by fitting a tube made of rubber around a tube made of steel, and press-fitting shafts, or rods, in both ends of the steel tube. Each of the shafts has a large diameter portion and a small diameter portion, the former being press-fitted in the steel tube. Another prior art platen comprises a cylindrical solid member made of aluminum alloy or the like, a tube made of rubber which is fitted around the solid member, and shafts which are press-fitted one in each of bores which are formed at both ends of the solid member. In any of such prior art platen configurations, because the shafts are produced independently of the steel tube or the solid member, or platen body, it is difficult to align the axes of the shafts with that of the platen body resulting in an increase in production cost. Further, the shafts which are press-fitted in the platen body adds to the weight of the platen and, thereby, bring about the need for a large output and expensive line feed motor which is adapted to drive the platen.
A sixth problem is related to backlash between intermeshing gears. In a printer of the type described, a platen is rotated, or indexed, to feed a paper to start a new line. Specifically, rotation of a line feed motor is transmitted to the platen by a motor gear which is mounted on the line feed motor, an idle gear, and a platen gear which is mounted on the platen. A prerequisite for the interline spacing to be maintained constant is that the platen be indexed by a constant amount and, therefore, the backlash between the motor gear had the idle gear and that between the idle gear and the platen gear be set up adequately. Usually, the platen is rotatably supported by side frames while the line feed motor is mounted to the side frames. Then, the idle gear is rotatably mounted to the side frame while adjusting both the backlash between the gear frame and the idle gear and the backlash between the idle gear and the platen gear. However, mounting the idle gear to the side frame while adjusting the two different kinds of backlash at the same time as stated is troublesome and inefficient. Moreover, such a procedure cannot readily establish adequate backlash, tending to lower the printing accuracy.
A seventh problem pertains to a paper feed mechanism. In a printing device of the type concerned, printing accuracy cannot be enhanced unless a paper is fed in tight contact with a platen. In light of this, a prior art printing device is provided with a deflector curved along a platen to guide a paper along the platen, feed rollers protruding toward the platen through slots which are formed in the deflector so as to urge a paper against the platen, a shaft on which the feed rollers are rotatably mounted, and an arm moving the shaft toward and away from the platen interlocked with the manipulation of a lever. When a paper is to be loaded on the platen, the feed rollers are retracted from the platen; after the paper has been loaded, the feed rollers are brought into contact with the platen to force the paper against the platen. This prior art implementation requires a disproportionate number of structural elements and, therefore, a disproportionate cost since the feed rollers are supported by the arm by way of the shaft.
An eighth problem also stems from the use of a die casting. A printing device of the type described has a housing which generally consists of a base cover and a top cover. The base cover is loaded with various electrical circuit elements and printing mechanisms while the top cover is mounted on the base cover in such a manner as to conceal them. The base cover has heretofore been implemented with a die casting which withstands painting for aesthetic purpose and desirably adapts itself to quantity production. However, as previously discussed, a die casting is thick and heavy and, moreover, expensive.
A ninth problem is as follows. A printing device of the type described involves various kinds of control and monitor processing which are based on time as typified by turn-on and turn-off control of LEDs representative of ribbon end and other errors as well as statuses of various switches arranged on a control panel, e.g., pose switch, energization control of buzzers, monitoring of switches disposed on the control panel and a rear panel as well as sensor switches, and monitoring of reception of data from a host machine. A prior art printing device includes exclusive timers for interface control so that such various kinds of control and monitoring which are based on time may be subjected to interface control. However, assigning one exclusive timer for interface control to each of the different kinds of control renders the construction complicated. When a single timer is shared by different controls, on the other hand, while any one of the controls occupies the timer the others cannot use the timer.